Magnetic transducer having an irregular nonmagnetic gap



May 3, 1949 H. A. HOWELL 2,469,266

MAGNETC TRNSDUGER HAVING AN IRREGULAR NONMAGNETC GAP Filed Aug. 14, 1946Patented May 3, 1949 MAGNETIC TRANSDUC'ER N "IRREGULAR NONMGNETC Giu'Hugh A. Howell, Valparaiso., Ind., Jassigner to The Indian-a SteelProducts Company, Chicago, vlll., a corporation ofllndiana ApplicationAugust 14, 1946, Serial No. 699,458

2 Claims. 1

This invention relates to a new magnetic recurdi-ng technique, and .moreparticularly to a variable gap magnetic :transducer fhead and :novelmethod.

IOne -of the `,principal features of the :present invention is V-toprovide Ean electromagnetic vtransducer head in which the nonmagnetic'gap thereof varies fromone side to another of the vgapx'so that the.flux density throughout 'the gap is notnniform but varies in apredetermined manner. When :the electromagnetic 'transducer .head .is so.arranged to :provide .a nonmagn'etic :gap which :the -density yvariesla predetermined :manner or pattern, 'a greatly :improved responsecha-'racteristic is obtained. Since 'the current 'ontput .fin the pickup-coil of a :magnetic :head `varies 1in Yaccordance with .frequency ofthe :sig-nal .impressed upon the record vmember and :in accordance 'withythe iengthof vthegap it is necessary in order to :get a level 'currentoutput to :have a fdiierent 'length 4gap for-each3frequency. By using:an elongated record medi-um having a relatively wide dimension, such asby using a :magnetic :tape and :lay varying the length of thenonmagnetic gap from one iside of the tape to thefother in `alongitudinal direction, the current k'output characteristic curve versus.frequency is `very greatly improved. IIndeed it has been :found thatIsuc'h an arrangement gives such a greatly improved result that areproducing headv may be connected directly into :any home .radioreceiver without any special equalizer circuit.

It is an yobject of the'present invention to pro vide a novel method andmeans for .magnetic recording `and reproducing.

Another obect of the present invention :is to provide a variable gapmagnetic transducer head fand method.

A further vobject of the pre'sent .invention is to provide :a noveltransducer head yin which 'the nonmagnetic gap is so arranged lthat theflux densi-ty throughout the gap varies from place to place.

A Still further object of th'e :present invention is to provide la'novel magnetic transducer head in which the 'confronting pole lfaces ofthe ipolar portions thereof fdo not Tlie vin :spaced .parallel 'planeslbut are of predetermined coniguration Vgiving `a yplurality ofldifferent distances rtif-:tween different confronting portions thereof.

Another and further object yof the present iinven'tion is `to provide anovel method rand means for recording signals on :a .magnetic tape insuch a .manner tmat the :so-called constant' current response is moreuniform and more easily corrected :j

2 equalization than tthe response hitherto Spossible.

The novel teatures which 'are fbelieved to be characteristic of my`invention are .set :forth `with paiiti'cul'arity in :the appendedclaims. My invention itself, however., rlootlfi as .to its 4orgam'zation.rand manner :of construction, together with further vobjects andadvantages thereof, .may best understood -by reference to the following:de- Jscription, taken in connection xwith the :accompanying fdrawmg,.in which:

Figure i vis la diagrammatic isometric View of an electromagnetictransducer head :embodying 'a inovel feature and characteristic fof 7thepresent invention;

.Figure '.2 is an A'enlarged .fragmentary isometric `view of ftheconfronting lfpolar portions of the :electromagnetic transducer `head:shown .in Figure 1;;

Figure 3 is a graph fsh'owing a typical constant current `response:obtainable from an electromagnetic transducer hea'd 'in whichytheconifronting pole faces lie in :spaced parallel Aplanes;

Figure 4 is fafgraphshowing -a .family-oi curves giving the variations.in the `frequency where the maximum output 'occurs with variations ingap lengths, each curve being -for -fa different `gap length;

Figure :5 is a :graph simi-lar to Figures 3 and 4 but giving :theresponse 4characteristic when Van yelect'roniagnetic transducer head isemployed rsuch as that illustrated in Figures -l and .2 "of the drawing;

Figure 6 is =a fragmentary isometric View fsimilar Ato Fig-ure i2 rbutillustrating a modified form of 4the present invention;

vFigure 7 is 'a iiragmentary isometric view 'similar to Figures y2 and'6 illustrating 'a third embodiment lof the present invention; fandEig-ure e8 is a .fragmentary iront elevational *view 'of the lpolarAportion :shown in 4Figure 7,.

The electromagnetic transducer head :shown diagrammen-cally in Figure 1:of the drawing :includes a core member Irl .a Ipair of -symphoniev:core (portions I2 .and 153 with :a nonmagnetic record member such as apaper tape coated with a permanent magnet material or a Steel tapehaving a reasonably high coercive force.

It will be remembered that in a conventional electromagnetic transducerhead in magnetic recording and reproducing devices the confronting polefaces Il and I8 lie in spaced parallel planes. With such prior artstructures the typical constant current response characteristic isillustrated in Figure 3 of the drawing. In this graph the curve I9 showsthe relative E. M. F. developed across the terminals 20 and 2l of thewinding I5 through the frequency range of 30 to 10,000 C. P. S., whenthe recording current which impressed the signal on the record member I6is maintained constant for all frequencies throughout the indicatedrange during the recording operation. From the curve I9 of the graph inFigure 3 it will be noted that the output rises beginning at the lowestfrequency at a rate of about 6 decibels per octave until a certaincharacteristic maximum has been reached.. The maximum as shown in Figure3 occurs at about 2,000 C. P. S. After the maximum is passed, the outputmore rapidly decreases. The rising portion of the curve I9 is due to thefact that the rate of change in flux induced in the transducer head Icwhen it is acting as a reproducing head by the tape I6 determines theoutput. The maximum rate of change for a constant total number of linesis nearly proportional to the frequency. For example a high frequencywould produce a aster rate of change than a low frequency at the sametape speed even though the remnant flux on the tape for the twofrequencies were of the same value. The characteristic drop after themaximum on the curve is due to a condition imposed by the relationbetween the gap length and the tape velocity. The particular frequencypoint where the maximum output occurs increases as the tape speed isincreased or as the gap is made shorter.

The characteristic rise in output from the lowest frequency to themaximum will be the range of most interest since it is in this rangethat the greatest amount of correction is required to get a relativelyflat response. It will be noted in Figure 3 that the output at 39 C. P.S. is about 28 db. lower than at the maximum. This means that in orderto make the proper correction an overall boost in the base response musthe produced in the recording and playback ampliner. A 14 db. boost inthe base response of the amplifier would record a higher level in thebase region and during playback there would be an equivalent 14 db.boost. It will be here noted that when the proper low frequency boost isused to flatten out the range immediately below the maximum, theresponse at the extreme low end is still down a few db. and the baseresponse suffers.

In Figure 4 there is a graph giving a family of curves showing thevariations in the frequency where the maximum output occurs withvariations in gap length, all other conditions being consideredconstant. More particularly the graph in Figure 4 gives four curves 22,23, 24, 25. The curve 22 is for the longest gap and the curve 25 is forthe shortest gap. By way of an example and not by way of limitation, thelongest gap may be .1" while the shortest gap may be .0015".

In accordance with the teachings of the present invention, a gap isemployed which varies in the length across the width of tape and as aresult thereof substantially the envelope of the family of curves shownin Figure 4 is obtained, as is shown by the full line 26 in Figure 5.The dotted lines in Figure 5 illustrate the same family of curves asshown in Figure 4. In this manner the maximum output is spread out orflattened over a substantial portion of the operating range. The curve26 of Figure 5 is obtained from an electromagnetic transducer headhaving a variable nonmagnetic gap as illustrated in Figure 1 of thedrawing. This particular head is shown more clearly in the fragmentaryview of Figure 2. More particularly the gap I4 is in the form of awedge. The pole faces I'I and I8 lie in vertical planes which divergefrom the back side of the core II to the front side of the core II. Thismay be seen clearly by the diverging lines 2t and 21 which are the topedges of the nonmagnetic gap.

Since the length of the nonmagnetic gap from one pole portion to theother, as shown in Figure 2, varies across the width of the tape it willbe seen that in effect we have the summation of an innite number ofcurves making up a family (of which a few are shown in Figure 4). Theoverall effect is that the current response is the summation of thefamily of curves and thus a current response characteristic like 2S isobtained rather than a response characteristic like The particular shapeof the nal response characteristic 26 as shown in Figure 5 of thedrawing may be controlled by the manner in which the gap length variesfrom one side of the core to the other. Thus instead of using a wedgeshaped gap which is not shown in Figures 1 and 2, a gap may be employedin which the upper lines 26 and 21 of the pole pieces I2 and I3 arecurves which gradually flare apart. As shown particularly in Figure 6,the initial portions may be parallel as at 28 and 29 before they startto gradually flare out, depending upon the shape of the final responsecharacteristic desired.

A third embodiment of the present invention is illustrated in Figures 7and 8 of the drawing wherein the top edges 26 and 2'I not only divergeas in Figure 2 of the drawing but the thickness of the pole pieces I2and I3 vary from one side to the other. This, when carried out in themanner shown in Figures '7 and 8 of the drawing, produces a much morerapid change in flux density in the gap I4 than is obtained with Figure2.

While I have shown some particular embodiments of my invention, it will,of course, be understood that I do not wish to be limited thereto, sincemany modifications may be made, and I, therefore, contemplate by theappended claims to cover all such modifications as fall within the truespirit and scope of my invention.

I claim as my invention:

l. A magnetic transducer head comprising a paramagnetic core having apair of confronting pole portions with a nonmagnetic gap therebetweenover which a traveling magnetizable tapelike record member is arrangedto pass, the confronting faces of said pole portions being shaped todiverge from each other from one side edge of said gap to the other andsaid pole portions also being decreased in thickness from said firstside edge to said second sidel edge, and a signal coil on said core forestablishing a fluctuating magnetic eld across said gap.

2. A magnetic transducer head comprising a paramagnetic core having apair of confronting REFERENCES CITED The following references are ofrecord in the nie of this patent:

Number FOREIGN PATENTS Country Date France June 13, 1924 Great BritainApr. 28, 1939

